Ignition timing in an internal combustion engine is typically controlled by a member of some type, such as a cam or distributor rotor, rotatably coupled to the engine crankshaft. In a multiple-cylinder engine for passenger car use, for example, the mechanical tolerances of the piston and crankshaft drive train may be controlled employing conventional technology such that piston motion may typically be located within 0.1.degree. of nominal. However, the tolerances of the rotating member and means such as cam switches or distributor points responsive thereto cannot readily be so closely controlled. Consequently, attempts at precise control of ignition timing to within tenths of a degree of crankshaft rotation and piston position have not been entirely successful.
It has been found desirable for many purposes to attempt control of ignition events with respect to the socalled piston top dead center or TDC position. Engine emissions and fuel efficiency, as well as ease of starting, depend in part upon initiating ignition at a precise point with respect to piston position within the associated cylinder. However, actual piston position typically is not directly determined. Rather, an indirect indication of piston position is derived from a rotating member coupled to the crankshaft, so the ignition timing accuracy is necessarily limited by the resolution capabilities of the rotating member and accuracy with which the rotating member is coupled to the crankshaft.
Timing of an engine often requires manual adjustment by an operator, as well as human judgemental considerations. For example, an operator may be expected manually to rotate a distributor housing while observing a mark or scribe line on the engine flywheel, harmonic balancer or camshaft pulley as illuminated by a strobe light. The mark on the flywheel may or may not be fairly accurately related to piston position. Such judgemental factors may easily lead to errors on the order of degrees. Additionally, the manual adjustment of the distributor is conducive to error, tampering and failure during operation.